The Galapagos is an evolutionary laboratory, a place where we can watch Darwin's theory of natural selection playing out in real time. It's not just different islands that host different species; on Isabela Island, the slopes of each volcano are home to a different species of tortoise, with the barren lava fields between them creating an impenetrable barrier to inter-breeding. On an expedition cruise to the Galapagos you will be amazed every day by stories like this.
There are a number of factors that have coalesced to make the Galapagos so unique. Firstly, there is the isolation of the archipelago, 600 miles from the South American mainland. Every species had to arrive here from somewhere else; for marine mammals and seabirds that's easy enough, but for land birds and reptiles the journey would have been extraordinary, brought here on 'rafts' of vegetation washed out to sea by storms. This is why you won't see many large animals on the islands, and why, for example, there are so many different species of finch, each adapted to fill a different evolutionary niche.
The second factor is the ocean currents, with the nutrient-rich Humboldt and Cromwell Currents upwelling around the islands, providing food for an amazing array of marine life and allowing species that are usually only found in cold water regions, such as penguins and albatross, to thrive on the Equator. The snorkelling here is like nowhere else on earth, and you'll feel like you've plunged into your very own Attenborough documentary as you swim amongst turtles, marine iguanas and Galapagos penguins.
The third element to consider is geology. The Galapagos is situated above a 'hot spot' where magma is forced upwards through the earth's crust, forming volcanoes and eventually islands. Over time, the earth's tectonic plates shift, so that older islands move east away from the hot spot, and new islands are formed in the west. This is why the western islands of Isabela and Fernandina, with their active volcanoes and black lava rocks, look so different to the lush highlands of Santa Cruz in the east.
An amazing 97% of the Galapagos is designated as a national park, and this remains one of the most pristine wildernesses on the planet. The wildlife is remarkably unafraid of humans, allowing you to get up close to species such as sea lions, land iguanas and bluefooted boobies. But even somewhere as remote and untouched as the Galapagos does not exist in isolation from the rest of the planet.
Invasive species brought to the islands by humans are a huge threat, from deliberate introductions such as dogs, which almost wiped out land iguanas on Santa Cruz in the 1970s, to accidental introductions such as rats brought in on ships, or the parasitic fly philornis downsii, which threatens a host of endemic bird species. The waters around the islands are affected by illegal trawling, plastic pollution and ocean acidification, and climate change poses a threat to species across the archipelago.
One consequence of visiting the Galapagos is that you will feel moved to protect the islands, and you will return home as an ambassador for their conservation. Organisations working to protect the Galapagos include the Galapagos Conservation Trust, based here in the UK; the Charles Darwin Foundation, which works in close partnership with the National Park; and new organisation Re:Wild, whose founding members include the actor Leonardo DiCaprio.